The invention relates to a device for picking up or displaying pictures, comprising a cathode ray tube having a target in an evacuated envelope and at least one cathode. The cathode emits electrons in an annular beam, and the tube includes at least one first grid having an aperture for passing the beam at a cross-over area in the beam.
In a device for picking up pictures the cathode ray tube is a camera tube and the target is a photosensitive layer such as, a photoconducting layer. In a device for displaying pictures the cathode ray tube may be a picture tube, whilst the target comprises a layer or a pattern of lines or dots of fluorescent material. Such a device may also be adapted for electronlithographic or electronmicroscopic uses.
Netherlands Patent Application No. 7905470 (corresponding to U.S. Pat. Nos. 4,303,930 and 4,370,797), open to public inspection, shows a cathode ray tube having a so-called "cold cathode". The operation of this cathode is based on the emission of electrons from a semiconductor body in which a pn junction is operated in the reverse direction in such a manner that avalanche multiplication of charge carriers occurs. Some electrons may then obtain as much kinetic energy as is required to exceed the electron work function; these electrons are then liberated on the main surface of the semi-conductor body and thus supply an electron current.
Since residual gases always remain in the evacuated envelope, negative and positive ions are liberated from these residual gases by the electron current. The negative ions are accelerated into the direction of the target. In the case of electrostatic deflection they may impinge upon a small area of the target and damage it or disturb its operation. Ion traps are used to prevent this harmful effect. An ion trap for negative ions is known, for example, from U.S. Pat. No. 2,913,612.
A proportion of the positive ions travels in the direction of the cathode under the influence of accelerating and focussing fields prevailing in the tube. If no special measures are taken, some of these ions will impinge on the semiconductor cathode and damage it.
This damaging effect may cause a gradual sputtering of a possibly present layer of material for decreasing the electron work function such as, for example, cesium. The emission properties of the cathode change owing to a re-distribution or even complete disappearance of this material. If this layer is not present (or is completely removed by the above-mentioned sputter mechanism) even the main surface of the semiconductor body may be attacked. In a semiconductor cathode employing avalanche multiplication of charge carriers, as described in Netherlands Patent Application No. 7905470, in which the emitting pn-junction is parallel to the main surface and is separated therefrom by a thin n-type surface zone, this surface zone may disappear completely as a result of this gradual sputtering, so that the cathode no longer functions. In a similar type of cold cathode as described in Netherlands Patent Application No. 7800987 (corresponding to U.S. Pat. No. 4,259,678) laid open to public inspection on 31 July 1979, the pn junction is exposed at the main surface of the semiconductor body. As a result of the above-described damaging effect of positive ions present in the electron tube, for example, the place where the pn junction is exposed on the main surface may change. This causes an unstable emission behaviour.
In a second type of cathode ray tube in which a pn junction is operated in the forward direction in the semiconductor cathode, the so-called negative electron affinity cathode (NEA-cathode), the emission behavior is also influenced because sputtering again takes place. Here too, the layer of material for decreasing the electron work function is first sputtered off gradually. Subsequently the pn-type surface zone of the cathode is attacked until the cathode no lnoger functions. Similar problems apply to other semiconductor cathodes such as, for example, the semiconductor cathodes as described in British Patent Applications No. 8133501 (corresponding to U.S. Pat. No. 4,516,146) and No. 8133502 (corresponding to U.S. Pat. No. 4,506,284).
It is found that the lifetime of cathode ray tubes manufactured with such semiconductor cathodes is considerably shorter owing to the above-mentioned processes.
A device of the type mentioned in the opening paragraph in which the annular emission pattern is obtained with the aid of a conventional thermionic cathode is known from French Patent Specification No. 1,361,143 (corresponding to U.S. Pat. No. 3,250,963).
A kind of sputtering may also take place in conventional cathodes, for example, with barium as a cathode material. It is true that the loss of barium is compensated by the supply of extra barium, but the electron emission becomes less stable owing to the inhomogeneous attack (sputtering) by the positive ions.